Gas atomization of a fluid such as a paint composition to break up the fluid into particles for subsequent application to a workpiece to be coated is a technique well recognized in the art. Spray apparatus generally employed is a spray gun to which is supplied a fluid stream and a gas stream. The gas is most usually air, but can, of course, be chosen from other gases as required. The fluid stream issues from the spray gun via a fluid nozzle while the gas stream issues via a gas nozzle, with the gas stream intersecting or otherwise disturbing the fluid stream to provide atomized sprayed fluid particles.
To improve coating characteristics of the fluid particles issuing from the spray gun, various techniques have been developed to electrostatically impart an electrical charge to these particles prior to their arrival on the workpiece to be coated. One such technique is induction charging. Briefly, and in relation to the instant invention, a method of inducing an electrical charge on sprayed fluid particles involves the placement of an induction charging electrode means adjacent the fluid and gas nozzles. This electrode means induces an electrical charge on the atomized fluid particles, which charge is opposite to the electrode's charge, as the particles pass within a charging zone created between the electrode means and the particle stream. The electrode means itself can be an integral fixture of the spray gun, or it can be removably connected to the spray gun. An example of the latter electrode means which can be fitted to a conventional spray gun is described in U.S. Pat. No. 4,009,829, to James E. Sickles, incorporated herein by reference.
It has been observed that some paint compositions have a strong tendency to break up unevenly when gas atomized. One example of such a group of paint compositions are the acrylic water-thinned automotive topcoat compositions. When such compositions are gas atomized, a group of large fluid particles (50-100+ microns) is formed while many fines (&lt;10 microns) are also produced. It has also been observed that induction charging tends to produce even more fines and to preferentially charge these fines while leaving the larger particles substantially uncharged. Since most of the paint mass is contained in the large particles, the expected benefits of electrostatic charging, such as increased transfer efficiency at the workpiece, are substantially reduced.
One object of the instant invention is, therefore, to provide an electrostatic spray gun that imparts a more completely charged fluid particle stream for subsequent application to the workpiece. Another object of the instant invention is to provide a charged fluid particle stream wherein the particles are more uniformly sized. These and other objects will become apparent within the body of this application.